Just take especially good care of them when they are poults. Once they hit 3 months they start to toughen up considerably.
There is some very good information on brooding/raising in the ALBC turkey manual: http://albc-usa.org/EducationalResources/turkeys.html
Like Frosty said, poults die from exposure, not drowning. Had a storm arrive, one July afternoon, while I was out adding sand to the turkey run. No rain, initially, just hail. The last turkey into the shed was me. Now, the `profile' presented by turks in heavy rain might have gotten some stories...